The present invention relates to a communication system, and more particularly to a system for a plurality of devices to perform communication with one another.
There are various connection systems for conducting serial data transmissions between a plurality of devices, especially between programmable devices incorporating a CPU. These systems have their own merits and demerits. Typical of such connection systems are as follows.
With the star connection system, a plurality of devices are each connected to a central device by a specific transmission line. Although having the advantage that the delay in data transmissions is small, this system has the drawbacks that the transmission line has a large overall length and that the entire system fails when the central device malfunctions.
In the daisy chain connection system, a plurality of devices are connected together in series. This system can be small in the overall length of the transmission line, but if an intermediate device malfunctions or the transmission line is broken at an intermediate portion, there arises the problem that the devices at opposite sides of the faulty device or portion become unable to communicate with each other.
The loop (or link) connection system, which is free of the above problems, has the advantage that there is a line bypassing an intermediate portion of malfunction. However, the system, which is generally unidirectional, still has the problem that the sending terminal and the receiving terminal need to be changed over by a fairly complex control device.
The so-called bus connection system also has the problem that if the bus line is broken at an intermediate portion, blocked communication will result.
With the ring bus connection system, all devices are capable of communicating with one another even if the ring bus line is broken, provided that the failure is limited to a single location.
Nevertheless, the bus connection system, even when in the form of a ring bus connection system, has the fatal drawback that it is not amenable to light communication. Light communication systems have found wide use in recent years because of various advantages. For example, they are operable satisfactorily in the presence of noises electromagnetically induced, and they have large transmission capacities. However, light communication systems can not be replaced by electrical communication systems in a simple fashion, for example, because of the problem of optical branch devices. Although already made available, such devices are presently expensive. With the bus connection system, each of the devices connected together requires an optical branch device. Further because the optical signal transmitted through the bus line is partly divided at the connection between the bus line and each device, the optical signal to be transmitted through the bus line needs to have a considerably great power.